The measurement of very high impedances can present difficulties. This is because the impedances involved are so large that the voltage applied to the device under test (DUT) is exceedingly large and/or the resulting current is exceedingly small. For example, using too high of a voltage can result in device breakdown or even arcing. In the case of capacitances, low frequencies exacerbate the problems, because the impedance is inversely related to the frequency.
Source measure units (SMUs) are well-known in the precision DC electrical measurement field for their ability to very accurately source a DC voltage signal and measure the resulting DC current signal or vice versa. For example, SMUs are available that can selectively source a DC voltage from a microvolt or less to a kilovolt or more and measure a DC current from an attoampere or less to an ampere or more (or vice versa). In DC measurement regimes, this permits the measurement of extremely high impedances (i.e., R=V/I).